THE EFFECT OF PHYTOPHAGOUS FISH ON THE CH EMICAL AND PHYSICAL-PROPERTIES OF WATER IN THE DANUBE RIVER BRANCH

The effect of a biomanipulative introduction of grass carp used in the removal of macro-vegetation on the selected elements of the ecosystem (carried out in 1982-1984) was examined. This influence had still bee n observed in the years 1985-1989. However, some indicators already sh owed the tendency to return to the 1981 values and some data already r eached that level. In 1985-1989 chemical and physical properties of wa ter in the Danube river branch also showed similar trends, even though there were seasonal and inter-year fluctuations probably due to the i nterference of the hydrological regime. Some parameters gradually retu rned back to the stage before grass carp introduction. In others the i nfluence of grass carp introduction remained present. Since 1985 or 19 86 the increase in average annual concentrations of P and organic N, a s well as the decrease in Ca content has stopped. On the other hand, a verage annual values of transparency were statistically significantly lower (except in 1987) in comparison with the level in 1981. The signi ficant increase in pH values in 1986 and Ca content and ammonium N in 1987 were recorded. The greatest differences in concentrations of sele cted indicators (in %) between the years 1988-1989 and the previous st age were observed in nitrate and ammonium N and insoluble substances. The highest decrease was recorded in phosphate P.